The present disclosure relates, in various embodiments thereof, to coated carrier particles. More specific, the disclosure relates to carrier particle having a coating that comprises polymethyl methacrylate and melamine. The carriers are suitable for use in developer compositions, which are suitable for use in imaging processes such as electrostatographic processes.
The electrostatographic process, and particularly the xerographic process, is known. This process involves, for example, the formation of an electrostatic latent image on a photoreceptor, followed by development of the image with a developer, and subsequent transfer of the image to a suitable substrate. Numerous different types of xerographic imaging processes are known wherein, for example, insulative developer particles or conductive developer particles are selected depending on the development systems used. Moreover, of interest with respect to the aforementioned developer compositions is the appropriate triboelectric charging values associated therewith, as it is these values that may enable continued formation of developed images of high quality and excellent resolution. In two component developer compositions, carrier particles are used in charging the toner particles.
Carrier particles in part are comprised, for example, of a roughly spherical or irregular shape core, often referred to as the “carrier core,” that may be generated from a variety of materials or purchased. The core is typically coated with a resin, such as a polymer or copolymer, and which resin may contain a conductive component, such as certain carbon blacks, to, for example, provide carrier particles with more desirable and consistent triboelectric properties. Including conductive components in the carrier coating may be disadvantageous in some instances, because it can, for example, be difficult and costly to blend the core and conductive component, and also the conductive component may not fully serve its purposes. For example, processes to incorporate conductive material into carrier coating include the use of electrostatic attraction, mechanical impaction, in situ polymerization, dry blending, thermal fusion and others, and which processes often result in only minimal amounts of conductive material being incorporated into the coating or generate conductive carrier coatings too large for effective and efficient use especially with smaller sized carriers. Additionally, dry blending processes and other mixing to incorporate the carbon black or other conductive material into the polymer coating can be selected. To avoid or minimize transfer of the carbon black from the polymer coating, however, the amount of carbon black that may be blended may be limited, for example, to 20 percent by weight or less, which limits the conductivity achievable by the resultant conductive polymer. Also, the carbon black from the carrier coating polymer can contaminate the toner resulting in changes in both charging performance and color of the toner, such as for example, a light colored toner, such as yellow.
In addition to the problems associated with loading conductive materials into coating resins, recent efforts to advance carrier particle science have focused on the attainment of conductive coatings for carrier particles to improve development quality and provide particles that can be recycled and that do not adversely affect the imaging member in any substantial manner. Many coatings can deteriorate rapidly, especially when selected for a continuous xerographic process where the entire coating may separate from the carrier core in the form of chips or flakes causing failure upon impact or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles, thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate.
A further problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity. High relative humidity hinders image density in the xerographic process may cause background deposits, lead to developer instability, and may result in an overall degeneration of print quality. In the science of xerography, the term “A Zone” refers, for example, to hot and humid conditions, the term “C Zone” refers, for example, to cold and dry conditions. Triboelectric charges are usually lower in the “A Zone” than in the “C Zone.” It is desirable to have the measured triboelectric charges (tribo) for a particular carrier in the A Zone and the C Zone, when entered into a ratio of A zone tribo /C zone tribo to be close to about 1 to obtain development in high humidity.
The use of powder coating processes has been a focus to coat carrier particles. Powder coating processes typically select polymers in the form of fine powders which can be mixed with a carrier core. The triboelectric charging value of the aforementioned carriers can be controlled by the polymer or mixture of polymers selected for the coating. But only a limited number of polymers are available in the form of fine powders, especially for the preparation of conductive carriers. Conductive polymers, which are in the form of fine powder, can be utilized as carrier coatings, for example a conductive carbon black loaded polymer, reference U.S. Pat. No. 5,236,629, the disclosure of which is totally incorporated herein by reference. Also, the carrier coating in some instances tend to chip or flake off, and fail upon impact, or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot usually be readily reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles, thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate. Furthermore, partially coated carriers have a short life, for example from about 1 to about 30 days, and poor stability.
A carrier coating commonly used is a polymethyl methacrylate (#MP-116 PMMA) available from Soken Chemical of Japan. This powder typically has a diameter of about 0.4 to 0.5 micrometer and it can be generated from polymethyl methacrylate. Usually, high amounts of PMMA are selected to coat a 30 to 50 micrometer carrier core and achieve a surface area coverage of about 85 to 95 percent on the carrier. The use of such high amounts of carrier coating often results in lower carrier yields because, for example, of the formation of fused aggregates. Fused aggregates usually need to be broken up or removed by screening. Crushing or breaking up of the aggregates may result in weak or “chipped off” areas on the carrier surface potentially causing poor coating quality. Screen separation may result in a lower yield as aggregates are removed from the final product.
Carrier particles for use in the development of electrostatic latent images are described in many patents including, for example, U.S. Pat. No. 3,590,000. These carrier particles may consist of various cores, including steel, with a coating thereover of fluoropolymers and terpolymers of styrene, methacrylate, and silane compounds.
There is illustrated in U.S. Pat. No.4,233,387, the disclosure of which is totally incorporated herein by reference, coated carrier components for electrostatographic developer mixtures comprised of finely divided toner particles clinging to the surface of the carrier particles. Specifically, there is disclosed in this patent coated carrier particles obtained by mixing carrier core particles of an average diameter of from between about 30 microns to about 1,000 microns with from about 0.05 percent to about 3.0 percent by weight based on the weight of the coated carrier particles of thermoplastic resin particles. The resulting mixture is then dry blended until the thermoplastic resin particles adhere to the carrier core by mechanical impaction, and/or electrostatic attraction. Thereafter, the mixture is heated to a temperature of from about 320° F. to about 450° F. for a period of about 20 minutes to about 60 minutes, enabling the thermoplastic resin particles to melt and fuse on the carrier core. While the developer and carrier particles prepared in accordance with the process of this patent are suitable for their intended purposes, the conductivity values of the resulting particles are not constant in all instances, for example, when a change in carrier coating weight is accomplished to achieve a modification of the triboelectric charging characteristics. Further, in regard to U.S. Pat. No.4,233,387, it is believed that only certain triboelectric charging values can be generated when certain conductivity values or characteristics are contemplated.
U.S. Pat. No. 4,937,166, the disclosure of which is totally incorporated herein by reference, describes a carrier composition comprised of a core with a coating thereover comprised of a mixture of first and second polymers that are not in close proximity thereto in the triboelectric series. The core is described to be iron, ferrites, steel or nickel. The first and second polymers are selected from the group consisting of polystyrene and tetrafluoroethylene; polyethylene and tetrafluoroethylene; polyethylene and polyvinyl chloride; polyvinyl acetate and tetrafluoroethylene; polyvinyl acetate and polyvinyl chloride; polyvinyl acetate and polystyrene; and polyvinyl acetate and polymethyl methacrylate. The particles are described to, for example, possess a triboelectric charging value of from about −5 to about −90 microcoulombs per gram.
U.S. Pat. No. 4,935,326, the disclosure of which is totally incorporated herein by reference, discloses a carrier and developer composition, and a process for the preparation of carrier particles with substantially stable conductivity parameters that comprises (1) providing carrier cores and a polymer mixture; (2) dry mixing the cores and the polymer mixture; (3) heating the carrier core particles and polymer mixture, whereby the polymer mixture melts and fuses to the carrier core particles; and (4) thereafter cooling the resulting coated carrier particles. These particulate carriers for electrophotographic toners are described to be comprised of core particles with a coating thereover comprised of a fused film of a mixture of first and second polymers which are not in close proximity in the triboelectric series, the mixture being selected from the group consisting of polyvinylidene fluoride and polyethylene; polymethyl methacrylate and copolyethylene vinyl acetate; copolyvinylidene fluoride tetrafluoroethylene and polyethylenes; copolyvinylidene fluoride tetrafluoroethylene and copolyethylene vinyl acetate; and polymethyl methacrylate and polyvinylidene fluoride.
U.S. Pat. No. 5,567,562, the disclosure of which is totally incorporated herein by reference, describes a process for the preparation of conductive carrier particles which comprises mixing a carrier core with a first polymer pair and a second polymer pair, heating the mixture, and cooling the mixture, wherein the first and second polymer pair each contain an insulating polymer and a conductive polymer, and wherein the carrier conductivity thereof is from about 10−6 to about 10−14 (ohm-cm)−1. The first polymer pair is preferably comprised of an insulating polymethyl methacrylate and a conductive polymethyl methacrylate, and the second polymer pair is preferably comprised of an insulating polyvinylidene fluoride and a conductive polyvinylidene fluoride.
There are illustrated in U.S. Pat. No. 6,042,981, the disclosure of which is totally incorporated herein by reference, carriers including a polymer coating wherein the polymer coating may contain a conductive component, such as carbon black, and which conductive component is preferably dispersed in the polymer coating. The conductive component is incorporated into the polymer coating of the carrier core by combining the carrier core, polymer coating, and the conductive component in a mixing process such as cascade roll mixing, tumbling, milling, shaking, electrostatic powder cloud spraying, fluidized bed, electrostatic disc processing or by an electrostatic curtain. After the mixing process, heating is initiated to coat the carrier core with the polymer coating and conductive component.
There are illustrated in U.S. Pat. No. 5,928,830, the disclosure of which is totally incorporated herein by reference, processes for the preparation of black and color toner compositions. This toner is prepared from latex comprised of a polymer core and a polymer shell thereover. The latex is fused or coalesced together forming toner particles comprised of an aggregation of the latex. Conductive or pigment components may be incorporated into the latex cores/shell prior to coalescence by blending a dispersion of the conductive or pigment component in a cationic surfactant with the latex core/shell in an ionic or nonionic surfactant. Specific examples of conductive components incorporated in U.S. Pat. No. 5,928,830 include conductive carbon black SC Ultra available from Conductex Inc. and antimony-doped tin oxide Zelec ECP3005-XC manufactured by E. I. DuPont.
In view of the foregoing, there is a need for coated carrier particles having high triboelectric charging characteristics. There is also a need for coated carrier particles having high conductivities. Further, there is a need for a coating composition that exhibits high coating coverage of relatively low coating loadings.